The present invention relates generally to invasive devices and methods for treatment of the heart, and specifically to devices and methods for electrical stimulation of the heart muscle.
Typical cardiac output for a healthy adult at rest is 5 liters per minute (LPM), with a heart rate of 70 beats per minute (bpm). The stroke volume, or cardiac output per beat, in this case is about 70 ml. During exercise, the heart rate can increase by up to a factor of three, and the stroke volume can increase by about 50%. Above a certain heart rate, however, the stroke volume begins to fall off due to insufficient filling of the ventricles and other factors. Consequently, cardiac output as a function of heart rate levels off at high heart rates and may even decrease at extremely high rates.
In heart failure patients, stroke volume is typically reduced, and the fall-off of stroke volume with increasing heart rate is generally more pronounced than in healthy individuals. Thus, less oxygenated blood is available to the body, making such high heart rates even less tolerable for heart failure patients. Furthermore, driving the heart at high rates, beyond the point at which the stroke volume has begun to fall off, reduces the amount of oxygenated blood available to perfuse the heart muscle via the coronary arteries, thus increasing the risk of myocardial ischemia.
PCT patent application PCT/IL97/00012, published as WO 97/25098, to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes methods for modifying the force of contraction of at least a portion of a heart chamber by applying a non-excitatory electric field to the heart at a delay following electrical activation of the portion. The non-excitatory, field is such that it does not induce action potentials in cardiac muscle cells, but rather modifies the cells response to at least the present activation. In the context of the present patent application, the use of such a non-excitatory field is referred to as Excitable Tissue Control (ETC). By suitably modifying the contraction of the heart, ETC can increase the stroke volume and thus enhance the cardiac output of patients suffering from heart failure or cardiac insufficiency due to other causes.
PCT patent application PCT/IL97/00235, whose disclosure is also incorporated herein by reference, describes a cardiac output controller using ETC stimulation. Control circuitry receives signals from one or more sensors, indicative of the heart""s activity, and responsive thereto, drives stimulation electrodes to provide the ETC stimulation to the heart. The effect of the controller on cardiac output is regulated by changing the timing of an ETC stimulation pulse relative to the heart""s activity, preferably relative to the heart""s electrical activity or ECG signals received by one of the sensors (which comprises a sensing electrode). Alternatively or additionally, the controller changes other pulse characteristics, such as the voltage, current, duration, polarity, waveform shape and frequency, and delay of the ETC pulse relative to a pacing pulse or to sensing of an activation potential in the heart. The sensors may also include flow rate sensors, pressure sensors, temperature sensors, oxygen sensors, and other types of sensors known in the art, so as to provide additional signals indicative of hemodynamic conditions, such as cardiac output, blood pressure or blood oxygenation.
It is an object of some aspects of the present invention to provide improved methods and apparatus for Excitable Tissue Control (ETC) of the heart so as to enhance hemodynamic performance thereof.
In preferred embodiments of the present invention, a cardiac stimulation device comprises one or more ETC electrodes, at least one sensor, and electronic control circuitry, coupled to the ETC electrodes and sensor. The ETC electrodes are placed at selected sites in the heart of a patient. The sensor is placed inside the patient""s body and generates signals responsive to the patient""s metabolic demand and/or physical exertion. Responsive to the signals, the circuitry drives the stimulation electrodes to provide ETC stimulation so as to enhance contractility of the heart muscle. Preferably, the stimulation is provided when the actual heart rate rises above a certain threshold, which generally indicates a metabolic requirement for increased cardiac output.
In some preferred embodiments of the present invention, the circuitry receives the signals from the sensor and analyzes the signals to determine a predicted, or normative, heart rate, corresponding generally to the expected beat rate of a healthy heart at a level of metabolic demand indicated by the sensor signals. The circuitry compares the predicted heart rate to the patient""s actual heart rate, which it preferably determines by analyzing signals received from electrodes coupled to the heart, most preferably from the ETC electrodes or from dedicated sensing or pacing electrodes. The circuitry drives the stimulation electrodes to provide the ETC stimulation particularly when the actual heart rate rises substantially above the predicted rate.
In some preferred embodiments of the present invention, the sensor comprises an accelerometer, and the signals generated thereby are indicative of the intensity of the patient""s physical activity or exercise. Additionally or alternatively the sensor comprises a respiration sensor, preferably a minute ventilation sensor, which indicates the patient""s average respiration rate and/or respiration volume. Preferably, the sensor or sensors are contained inside a can, which also contains the control circuitry and is implanted in the patient""s body. It will be understood, however, that any other suitable sensor or combination of sensors may be used, and that one or more of the sensors may be outside the can, preferably located on, in or near the patient""s heart.
In some preferred embodiments of the present invention, upper and lower bounds of the heart rate are established, wherein outside these bounds, the ETC stimulation is typically not applied for reasons of patient safety, or because the stimulation is not required. Within these bounds for any given predicted heart rate, upper and lower threshold values of the actual heart rate are defined which are typically, although not necessarily, respectively greater than and less than the predicted rate by predetermined factors. When the heart rate increases above the upper threshold value (without exceeding the upper bound), the ETC stimulation is initiated. When the heart rate drops below the lower threshold value, the ETC stimulation is discontinued. Between the upper and lower threshold values, a hysteresis function is applied to prevent on/off oscillation of the stimulation.
Preferably, an intensity level of the stimulation is also adjusted responsive to the actual and/or predicted heart rate, and/or to the comparison of the actual and predicted rates. Most preferably, the stimulation intensity is adjusted by varying a duty cycle of ETC stimulation pulses relative to the heartbeat. This and other suitable methods for varying and adjusting the intensity are described in Israel Patent Application 127,092, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference.
The present invention thus improves safety and long-term efficacy of the ETC stimulation, by providing enhancement of cardiac contractility, and hence of hemodynamic performance, when and as indicated by the patient""s metabolic needs. Controlling the ETC stimulation by this method also reduces power consumption by the device and consequently increases battery lifetime when the device is implanted in the patient""s body.
Although preferred embodiments of the present invention are described in terms of certain specific types of sensors and certain methods of applying and controlling the ETC stimulation, it will be understood that the scope of the present invention is in no way limited to these modalities. The principles of the present invention may be applied using any other suitable types of sensors, ETC modalities and methods of controlling ETC stimulation, including (but not limited to) those described in the above-mentioned PCT and Israel patent applications, as well as in the U.S. patent application Ser. Nos. 09/101,723 and 09/254,902, which correspond to the PCT applications and are likewise incorporated herein by reference. Furthermore, the principles of the present invention may be adapted for use in conjunction with pacing of the heart, as described, for example, in PCT patent application PCT/IL97/00236, in the corresponding U.S. patent application Ser. No. 09/254,900, or in Israel patent application 125,424, all of which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for modifying contractility of the heart of a patient, including:
receiving signals from a sensor coupled to the body of the patient indicative of physical exertion by the patient;
analyzing the signals to estimate a metabolic demand of the patient; and
applying excitable tissue control (ETC) stimulation so as to enhance contractility of the heart muscle responsive to the metabolic need.
Preferably, receiving the signals includes receiving a signal responsive to motion of the patient most preferably receiving an accelerometer signal. Alternatively or additionally, receiving the signals includes receiving a signal responsive to respiration of the patient.
Preferably, the method includes measuring the patient""s actual heart rate, wherein applying the signals includes comparing the actual heart rate to the metabolic demand and applying the stimulation responsive to the comparison. Most preferably, analyzing the signals includes predicting a normative heart rate responsive to the metabolic demand, and comparing the actual heart rate to the metabolic need includes comparing the actual and normative heart rates.
In a preferred embodiment, comparing the actual and normative heart rates includes:
for a given value of the normative heart rate, specifying a trigger threshold heart rate above the normative rate, and a turnoff threshold heart rate below the normative rate, thereby defining a hysteresis band intermediate the trigger and turnoff rates,
wherein applying the stimulation includes initiating the stimulation when the actual heart rate passes above the trigger rate, and suspending the stimulation when the heart rate passes below the turnoff rate.
Preferably, applying the stimulation includes controlling an intensity of the stimulation responsive to the metabolic demand.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for modifying contractility of the heart of a patient, including:
specifying a trigger threshold heart rate and a turnoff threshold heart rate, which is substantially below the trigger threshold heart rate, thereby defining a hysteresis band intermediate the trigger and turnoff rates;
monitoring the patient""s heart rate;
initiating excitable tissue control (ETC) stimulation so as to enhance contractility of the heart muscle when the heart rate passes above the trigger rate; and
suspending the ETC stimulation when the heart rate passes below the turnoff rate.
Preferably, the method includes specifying an upper tracking rate above the trigger threshold rate and suspending the ETC stimulation when the heart rate passes above the upper tracking, rate.
There is also provided in accordance with a preferred embodiment of the present invention, apparatus for stimulating cardiac tissue in the body of a patient, including:
at least one sensor, coupled to the body which generates signals indicative of physical exertion by the patient;
one or more electrodes which are placed in contact with the heart, and
an electrical control unit, which receives and analyzes the signals from the at least one sensor so as to estimate a metabolic demand of the patient and which applies excitable tissue control (ETC) stimulation to the electrodes so as to enhance contractility of the heart muscle responsive to the metabolic demand.
Preferably, the sensor generates a signal responsive to motion of the patient. Most preferably, the sensor includes an accelerometer. Alternatively or additionally, the sensor includes a respiration sensor.
Preferably, the control unit monitors the patient""s actual heart rate, compares the actual heart rate to the metabolic demand and applies the stimulation responsive to the comparison. Most preferably, the control unit predicts a normative heart rate responsive to the metabolic demand, and compares the actual and normative heart rates so as to control application of the stimulation.
In a preferred embodiment, for a given value of the normative heart rate, the control unit determines a trigger threshold heart rate and a turnoff threshold heart rate responsive to the normative rate, the trigger threshold heart rate being higher than the turnoff threshold heart rate thereby defining a hysteresis band intermediate the trigger and turnoff rates, and
the control unit initiates application of the ETC stimulation when the actual heart rate passes above the trigger rate, and suspends the application of the stimulation when the heart rate passes below the turnoff rate.
Preferably, the control unit controls an intensity of the stimulation responsive to the metabolic need.
There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for modifying contractility of the heart of a patient, including:
one or more stimulation electrodes, which are placed in contact with the heart; and
a control unit which monitors the patient""s heart rate, and which specifies a trigger threshold heart rate and a turnoff threshold heart rate, which is substantially below the trigger threshold heart rate, thereby defining a hysteresis band intermediate the trigger and turnoff rates and
which initiates application of excitable tissue control (ETC) stimulation to the electrodes, so as to enhance contractility of the heart muscle, when the heart rate passes above the trigger rate, and suspends the ETC stimulation when the heart rate passes below the turnoff rate.
Preferably, the control unit specifies an upper tracking rate above the trigger threshold rate and suspends application of the ETC stimulation when the heart rate passes above the upper tracking rate.